The present invention relates to a subcarrier generating device for a color signal processing circuit in a color television receiver, and in particular, to a device ideally suited for use in semiconductor integration.
Accompanying the advancements in recent years of the integration technology of signal processing circuits, there has also been a trend toward the integration of color signal processing circuits in color television receivers, including subcarrier generating devices. Such subcarrier generating devices include, for example, a voltage controlled oscillator (VCO), a tint circuit, and a subcarrier amplifier. The VCO generates a continuous subcarrier signal. This continuous subcarrier signal is supplied to the tint circuit as a reference carrier. The tint circuit is connected to a variable resistor used for varying the phase. Accordingly, the phase of the supplied reference carrier is controlled by the adjustment position of the variable resistor. This phase-controlled reference carrier is supplied to the subcarrier amplifier. The subcarrier amplifier shifts the phase of the supplied signal and further performs phase composition, thus providing each type of continuous subcarrier signal such as the carrier R-Y CW used for R-Y signal demodulation and the carrier B-Y CW used for B-Y signal demodulation. These R-Y CW and B-Y CW are supplied to the demodulation circuit. In the circuit, these signals are demodulated in cooperation with the chroma signal, and the R-Y CW and B-Y demodulation-axis CW signals. The R-Y and B-Y demodulated outputs are obtained as a result. Then the R-Y and B-Y demodulated outputs are matrixed, and the G-Y demodulated output is also obtained.
In such a conventional circuit, adjustment of the variable resistor changes the phase of each of the R-Y and B-Y CW signals for the tint adjustment. However, at this time, the relative phase of the R-Y CW and the B-Y CW must be maintained at a fixed value (e.g., at 90.degree. ). If there is a shift in this relative phase, the colors on the screen will become unnatural. In addition, because the demodulation of the G-Y signal is obtained by matrixing the R-Y and B-Y demodulated outputs, a shift in the relative phase will bring about a change in the amplitude of the G-Y signal or a change in the relative phase between the G-Y signal and other signals, thus causing the color to become even more unnatural.
In the conventional circuit as described above, when harmonic signals in addition to the fundamental signal are included in the reference carrier signal from the VCO, the adjustment position of the variable resistor will bring about a shift in the relative phase of the R-Y CW and B-Y CW. That is to say, when harmonic signals are included in the output of the VCO, and the variable resistor in the tint circuit is adjusted to adjust the tint, when the variable resistor is at the center value (tint center), when it is at maximum (tint max.), and when it is at minimum (tint min.) values, the relative phase between the subcarrier R-Y CW and B-Y CW will differ. In addition, changes of amplitude and relative phase will appear in the G-Y demodulated output obtained by the matrixing of the R-Y demodulated output and the B-Y demodulated output. As a result of such factors, there will be a deterioration of the color of the reproduced image displayed on the screen of the television receiver, and therefore an unnatural image.